222 



Mr. E. Wilson. The Growth of Magnetism in 



Suppose a transformer core to be built up of 1 mm. wires, or J mm. 

 plates, insulated from one another, the transformer being in action 

 with no currents in its secondary circuit. The reaction of the core 

 upon the primary or magnetising coil will be the rate of change of the 

 average induction over the whole core. The average induction per 

 sq. cm. of a particular wire or plate will differ from the induction per 

 sq. cm. at the surface of such wire or plate by an amount varying with 

 the frequency and with the value of B at the surface. For high and 

 low values of the surface B and a given frequency the average over the 

 whole wire or plate differs less from the maximum at the surface than for 

 intermediate values of the surface B. The relation between the perme- 

 ability of the iron and the rate of propagation of magnetism in the iron 

 has been explained in the case of simple reversals,* and agrees with 

 what we have just observed. When the limits of B are small, that is, 

 the permeability is small, the magnetism is propagated rapidly. For 

 intermediate values of the limits of B, that is, when the average 

 permeability is large, the rate of propagation is small. With the high 

 limits of B the average permeability is small and the magnetism is 

 propagated more rapidly. Setting aside the subject of magnetic 

 viscosity, we should expect the average B over the whole wire or plate 

 to be equal to the surface B if these induced currents did not exist. 

 The curves show that for a given frequency there is an effect which 

 increases the extent to which equalisation of the induction density over 

 the core may be carried according as the maximum limits of B at the 

 surface are on the lower or higher part of the curve of induction of the 

 material. The dissipation of energy, due to magnetic hysteresis and 

 induced currents, will likewise be affected since uniform distribution 

 gives minimum dissipation for the same maximum average induction 

 over the whole core. 



Not only have we to consider the maximum value of the induction 

 density at different parts of the core, but the phase of such induction 

 density. It is not necessary to publish all the curves obtained, but as 

 an example one might contrast in figs. 7 and 8 the curves of E.M.F. 

 obtained with periodic times of 10*3 and 2-6 minutes for about the 

 same maximum magnetising force, namely, 9*6 and 9*5. In figs. 7 and 

 8 the E.M.F. curves are plotted to a scale giving C.G.S. units per 

 sq. cm. of the area embraced by the respective coils, the curve number 

 corresponding with the coil number in fig. 2. With 10 minutes' 

 periodic time the induction is practically reversed over the whole core 

 by the time the current has attained its maximum value ; whereas 

 with 2'6 minutes' periodic time the current is again zero when the 

 innermost coil (No. 1) is experiencing its maximum E.M.F. In the 

 first case nearly the whole of the change for each coil aids the average 



* Hopkineon and Wilson, ' Journal of the Inst. Elec. Eng.,' vol. 24, p. 195. 



